In a yarn feeder as known from GB-C-1355518 (FIG. 9) the yarn control element is formed as a stable hollow body supported separately from the storage drum and put over the withdrawal end at the front side of the storage drum. The yarn control element surrounds the cylindrical circumferential section of the storage drum with radial distance by its big diameter cylindrical end portion. In this case the yarn control element minimizes the formation of a balloon during withdrawal of the yarn from the yarn store on the storage drum overhead of the withdrawal region into a withdrawal eyelet positioned in front of and co-axially relative to the storage drum. Among other influences the yarn orbiting around the withdrawal region due to centrifugal force tends to separate in an arch shape from the circumference of the storage drum and to form a spiral shaped ballooning-configuration downstream into the withdrawal eyelet. This known phenomena of the balloon formation leads to strong mechanical loads in the yarn, to an increasing tension in the withdrawn yarn and to yarn breakages, respectively. These disadvantages are eliminated by suppressing the balloon formation by means of the yarn control element. However, due to its distance from the circumferential surface of the storage drum the yarn control element does not suffice to guarantee a uniform and pre-determined basic tension in the withdrawn yarn. For this reason and as mentioned in GB-1355518 in addition to and upstream of the yarn control element frequently a so called brush braking ring is installed at a bracket fixed to the housing, the bristles of which contact the drum and brake the yarn in order to guarantee the necessary yarn withdrawal tension. However, a brush braking ring has, for mechanical reasons, the disadvantage of a speed-dependent braking effect i.e. a braking effect raising the yarn tension with increasing speed. This is a disadvantage, since the withdrawal tension should remain approximately uniform within a broad speed range in order to achieve optimal insertion relations in the weaving machine.
It is an object of the present invention to create a yarn feeder of the kind as disclosed above in which in a structurally simple way an essentially constant yarn withdrawal tension can be achieved in conjunction with the balloon limiting function. This object can be achieved by providing a yarn control element configured as a finite band having a planar cross section, which band is laid on the cylindrical circumferential section with mutually aligned adjacent ends. The band is resistant against extension in circumferential direction of the storage drum and is yieldable in the radial direction of the storage drum, and an elastic tensioning device is provided which preloads the band against the circumferential section.
The yarn is pulled through between the lower side of the band and the cylindrical circumferential section of the storage drum and in addition orbits in the circumferential direction during its withdrawal movement. The mechanical obstacle of the band suppresses the balloon formation tendency of the yarn extremely effectively. Further, a uniform and precisely pre-determinable braking effect is exerted onto the yarn by the contact pressure of the band against the circumferential portion, which braking effect surprisingly remains constant with yarn speed variations. In this way the balloon formation is suppressed by the yarn control element and simultaneously an essentially constant yarn withdrawal tension is achieved. The yarn has to overcome the elastic pre-load spanning or forcing the band onto the circumferential section only at its passing location below the band which in the circumferential area of the passing location remains supported on the circumferential section. Thanks to the yieldability of the band in the radial direction the yarn is forming so to speak an orbiting wave in the band or a sickle shaped free space, respectively, such that at this deformation the deformation resistance of the band remains essentially constant and independent of the speed. Since the perpendicular force on the yarn resulting from the pre-load of the band against the circumferential section remains essentially constant and is independent of speed, and due to the constant friction coefficients between the yarn and the band and the circumferential section, respectively, the advantageous result is that the yarn withdrawal tension remains essentially constant. The reasons for this positive effect of the yarn control element cannot be judged precisely. However, the result of the co-operation between the yarn, the band and the circumferential section, namely an essentially constant yarn withdrawal tension, is convincing. Unexpectedly even the gap between the ends of the band does not disturb the uniform braking effect, apparently since due to the dynamic movement relations the yarn does not feel the circumferential interruption in the band, provided that the yarn direction is inclined in relation to the direction of the circumferential gap.
Suitably the band is of flexible metal or plastic. Furthermore the use of a compound-band is possible having a wear resistant layer and in connection therewith another layer with differing properties.
The band in one embodiment includes a circumferential interruption so that two terminal ends are formed. The terminal ends of the band are parallel to one another and inclined, and the circumferential interruption in the band is as small as possible. The direction of the ends of the band is selected such that the yarn never will pass across the circumferential interruption with parallel orientation.
The band can be elastically pre-loaded by a tensioning device and is positioned on the circumferential section exclusively by frictional contact. The friction at the circumferential section is used to suppress an axial wandering of the band under the drag of the yarn.
The band can be positioned on the storage drum by means of a circumferentially extending shoulder. In this way bands of random widths can be positioned safely and with low tension for sensitive yarn materials.
In one embodiment, the band together with the tensioning device is supported in a holder and is positioned on the circumferential section by means of the holder. The holder, which is oriented parallel to the axis of the storage drum, is adjustable and is mounted in a bracket which is fixed to the housing and extends along the circumference of the storage drum. Thus, the position of the band is assured by the holder, suitably allowing an adjustment of the band in the longitudinal direction of the storage drum, and the holder also takes up occurring axial forces.
At the yarn entrance side of the band, a circumferentially extending depression is provided in the storage drum circumference, or the cylindrical circumference section is configured to project outwardly beyond the storage drum circumference at the yarn entrance side. In accordance with this embodiment, a significantly gentle yarn entrance below the band is achieved.
The tensioning device is provided with a spring element which at least bridges the circumferential interruption between the ends of the band. The spring element is connected with the band at least locally, and preferably is constructed of a tensioned elastomeric material, rubber strip or a tension spring. In this embodiment, tension is brought into the band exclusively by the spring element which at least bridges the circumferential gap of the band and pulls both ends of the band towards each other and produces in this way radial contact pressure.
In the alternative embodiment, the tension and the contact pressure of the band can be varied by means of a tension adjustment device in order to adapt to different withdrawal conditions or different yarn materials
In yet another embodiment the tensioning device is an annular magnet provided in the storage drum. Thus, the band is radially pressed against the circumferential section by magnetic effects.
Alternatively a circumferentially closed tension ring serves to span the band. The tension ring suitably is made from a material which does not produce a significant resistance against the wave shaped deformation of the band due to the influence of the yarn.
In another embodiment, the tensioning ring is open and includes overlapping ends. The tension ring spanning the band uniformly about the circumferential section can thus be adjusted in its length in order to vary the tension.
The tension ring can be made of plastic foam material or rubber, or may be formed as a ring-shaped coil spring. This construction is particularly advantageous since plastic foam material, elastomeric material, or rubber or even a ring-shaped coil spring lead to constant spring properties for long durations, and are contamination-proof. Particularly, homogenous materials have a negligible resistance against the wave motion of the band.
In an alternative embodiment the tension ring is designed as an annular membrane of rubber or elastomeric material.
In yet another alternative embodiment, the tension ring is in the form of a ring body with spokes or teeth. This type of tension ring simultaneously can be used to position the band in the axial direction.
A particularly advantageous embodiment includes a tension ring configured as a hollow, tubular and inflatable ring. The ring tube per se is able to produce a circumferential tension. Depending upon the degree of inflation, the tension or the radial pressure, respectively, can be varied and adjusted precisely, even without a tension adjustment device.
In another embodiment the band is solely fixed by friction, namely with the counter pressure of the storage drum.
In still another alternative embodiment the band at least in sections is connected with the tension ring. This can be of advantage for mounting reasons. In case of foam material, rubber or elastomeric material the connection can be made by in-situ forming, vulcanisation or bonding and for that reason can be made very uniformly such that a completely uniform response behaviour of the band is achieved for the passage of the yarn.
The tension ring can be as broad as the band, in which case a uniform back-up contact is achieved over the width of the band. Since the band already is rigid against bending in lateral direction due to its shape and due to the contact with the storage drum the tension ring may even be made narrower than the band.
In another embodiment the band is supported in the holder via the tension ring or by means of a carrier ring surrounding the tension ring. No special positioning of the band on the storage drum is needed.
In yet another embodiment, the tension adjustment device is provided at the holder or at the carrier ring of the tension ring. The variation of the tension of the band is possible by increasing or reducing the circumferential length of the holder or the carrier ring of the tension ring such that the tension ring can be biased more or less.